1. Technical Field
The present invention relates to a closely arranged multicore optical fiber for use in a thermal lens type optically controlled optical switch in the field of optical communication and optical information transmission.
2. Related Art
General single-mode optical fibers include a core and a clad having different refractive indexes in such a structure that the core formed of a core member having a relatively higher refractive index is surrounded by the clad formed of a clad member having a relatively lower refractive index. Accordingly, the cross section of the core has a circular shape and that of the clad has a doughnut shape. Further, “holey fibers” formed by “air-holes” (having an extremely low refractive index) surrounding the periphery of the substantial portion of the core through which light travels, rather than the fibers using a clad and a core having different refractive indexes, have been developed. In both the core/clad single mode optical fiber and the holey fiber, the diameter of the core portion through which light transmits is approximately several μm to 10 μm, whereas the outer diameter or the diameter of a whole single optical fiber is normally 100 μm or more. (See “Beginners Guide to Optical Fiber Communication (revised 3rd edition)” by Yasuharu SUEMATSU and Kenichi IGA, Ohmusha, 1989.) These diameters are determined so as to maintain the strength of the fibers for preventing disconnection from the manufacturing viewpoint, rather than under optical requirements. If the optical fiber is manufactured with extreme care, the diameter of a single optical fiber could be reduced to approximately 80 μm.
On the other hand, in the field of optical communication and optical information transmission, a “multicore (multiwire) optical fiber” generally refers to an optical fiber formed of a simple bundle of general optical fibers. In general, a multicore optical fiber in which each fiber includes clad with a diameter of 125 μm is being widely used, both in a single-mode and a multimode. Japanese Patent No. 2781710 discloses, in the specification, a manufacturing method of a multi-waveguide type cylindrical optical conductor for a communication cable, with a view to improving a compact optical cable and providing a manufacturing method which facilitates manufacture of a multi-waveguide type cylindrical optical conductor for a communication cable. Specifically, the conductor includes a plurality of cylindrical base optical fibers buried within a cylindrical silica block, each cylindrical base optical fiber being formed of a core and a clad, and all the cores of the base optical fibers are regularly arranged on a generatrix of a cylindrical body having an axis in common with the cylindrical block. The manufacturing method includes the following steps: forming a preform from a cylindrical silica rod, together with a plurality of cylindrical base preforms, in which the cylindrical silica rod has a plurality of grooves running along the generatrix on the outer surface thereof, each of the cylindrical base preforms is formed of core glass surrounded by cladding glass and is partially housed within the cylindrical silica rod, and the preform includes the base preforms in the number corresponding to the number of base fibers to be formed and has the same shape as that of the conductor to be manufactured; applying silica particles to the preform formed in this method using a plasma technology covering method, to give the preform a uniform cylindrical outer surface shape; and performing wiredrawing with respect to an assembly thus obtained to obtain the conductor having the base fibers buried in the silica block. The specification further describes that the transverse dimension of the base preform is selected such that the base fibers each including a core having a diameter of approximately 8 μm to 10 μm and a clad having an outer diameter in the range between 25 μm and 35 μm can be obtained after wiredrawing of the fibers. In the multicore optical fiber (referred to as a “multiguide cylindrical conductor” in the specification of JP 2781710 B) manufactured by the above method, which is manufactured with the base fibers being bound together at both ends as is obvious from the manufacturing method, it is practically impossible to handle the individual optical fibers (a waveguide cylindrical optical conductor) independently at one end, e.g. to connect each of the individual fibers to a single optical fiber separately.
The present inventors disclose, in PCT/JP2007/053707, that a plurality of adjacent single-mode optical fibers are closely arranged in parallel to each other and bound together, with the distance between centers of the adjacent optical fibers being no greater than a predetermined value (approximately 40 μm) at one end, and control light is caused to emit from one end and signal light is caused to emit from the other end, thereby allowing an extremely efficient operation of a thermal lens type optically-controlled optical path deflection switch.
The present invention advantageously provides an end surface closely arranged multicore optical fiber and a manufacturing method thereof, in which at least two core/clad single mode optical fibers including a clad surrounding the outer periphery of a core are closely arranged in parallel to each other and bound together at least at one end of the individual single mode optical fibers, with the core diameter being maintained and the outer diameter of the clad being reduced, and the individual optical fibers are individually independent at the other end of the individual optical fibers. With the above structure, the individual optical fibers (waveguide cylindrical optical conductors) are bound together at least at one end, and, at the other end, are handled individually and separately, e.g., can be individually connected to separate single optical fibers.